I think the next handheld will approximate Wii U/PS3 style visuals at 960x540 resolution on a 1280x720 screen (lesser intensive games will run at full screen res).
The "console" won't really be a "console" but the handheld chip put into a small AppleTV sized box (no disc drive), with 2x-3x the CPU/GPU cores (overclocked perhaps) + 2-3x the RAM. It will be able to run the same games as the handheld just a full screen 1080p resolution (Wii U level graphics) with a few spiffier effects.
It will be extremely cheap (lower than $179.99 or so at launch) and have a ton of Virtual Console games. Both will have an NFC reader built-in for amiibo toy compatibility.
Games will come on 3DS style cartridges, just with higher storage capacity (8GB-32GB). Or you can download them from a new unified eShop.
If you buy both you'll be able to play the same games at home or on the go through Cloud saves.
